1. Field of the Invention
The invention is directed in general to vehicle brake pad wear sensors and more specifically to a sensor that is able to measure and display the actual amount of wear left on a brake pad at any given time.
2. Discussion of Related Art
Typical brake pad wear sensors use a simple electrical contact to switch on a warning indicator. When the contact is exposed to an electrically grounded brake rotor or drum after a certain amount of wear on the pad, a circuit is closed and current flows to a warning light. Other types of sensors sound an acoustic alarm when a portion of the sensor rubs against the drum or rotor after the brake pad wears a set amount. These warning devices do not indicate the amount of wear left on the brake pads, only that the pads need to be replaced. Because an uncertainty of when brake pad failure will actually occur exists, the designed brake pad life may be cut short due to premature brake pad replacement. In addition, the harsh environment in which brakes must operate often leads to failure of indicators that locate electrical circuitry or sensitive components close to the working brake parts.
Some prior sensors involve the use of capacitance in some manner to indicate brake wear. U.S. Pat. No. 6,105,422 discloses a brake tester having capacitive proximity sensors extending from a sleeve concentrically disposed about a brake shoe assembly. When the brake is applied and the brake shoes advance toward the sensors, the sleeve simulates a brake drum surface. A central processing unit receives signals from each sensor indicating the presence of the brake shoe. If the shoe does not contact the simulated drum surface, there is no signal from the adjacent sensor. The specific contact area between the brake shoe and drum surface can be identified, correct brake assembly can be confirmed and premature wear can be predicted. The device needs a large number of sensors to be accurate, does not indicate progressive wear of the brake shoes, and requires relatively expensive components such as the sensors themselves and the central processing unit.
In U.S. Pat. No. 5,632,359, the state of wear of a brake pad is monitored by a capacitive sensor. The sensor periodically measures the capacitance between contacts corresponding to relative positions of a brake caliper and a brake carrier connected to the earth potential of the vehicle. The capacitive measurement is compared to a predetermined threshold and a signal is sent based on the comparison result. Because of the number of moving brake components involved, the actual capacitive measurement may not provide a true indication of brake pad wear. This brake wear indicator also requires a microprocessor and relatively complex circuitry to process the capacitive measurement and decide on and generate the warning signal. In this age of increasingly complex and expensive electrical circuitry being placed in vehicles for purposes such as navigation and entertainment, it is important that design attention remain focused on driver and passenger safety. But safety devices should still be engineered to be simple, cost-efficient and failure-proof, and many typical brake pad wear sensors do not meet these standards.